High-power external-cavity optically-pumped semiconductor lasers

ABSTRACT

External-cavity optically-pumped semiconductor lasers (OPS-lasers) including an OPS-structure having a mirror-structure surmounted by a surface-emitting, semiconductor multilayer (periodic) gain-structure are disclosed. The gain-structure is pumped by light from diode-lasers. The OPS-lasers can provide fundamental laser output-power of about two Watts (2.0 W) or greater. Intracavity frequency-converted arrangements of the OPS-lasers can provide harmonic laser output-power of about one-hundred milliwatts (100 mW) or greater, even at wavelengths in the ultraviolet region of the electromagnetic spectrum. These high output powers can be provided even in single axial-mode operation. Particular features of the OPS-lasers include a heat sink-assembly for cooling the OPS-structure, a folded resonator concept for providing optimum beam size at optically-nonlinear crystals used for frequency conversion, preferred selection of optically-nonlinear materials for frequency-conversion, and compound resonator designs for amplifying second harmonic-radiation for subsequent conversion to third or fourth harmonic radiation.

PRIORITY INFORMATION

This application is a continuation of application Ser. No. 10/695,583,filed Oct. 28, 2003 now U.S. Pat No. 7,180,928, which is in turn acontinuation of application Ser. No. 10/180,825, filed Jun. 26, 2002,now U.S. Pat. No. 6,683,901, which is in turn a continuation ofapplication Ser. No. 09/938,226, filed Aug. 23, 2001, now U.S. Pat. No.6,438,153, which is in turn a continuation of application Ser. No.09/263,325, filed Mar. 5, 1999, now U.S. Pat. No. 6,285,702.

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to external-cavityoptically-pumped semiconductor lasers (hereinafter, OPS-lasers)including a surface-emitting, semiconductor multilayer (periodic)gain-structure. The invention relates in particular to arrangements ofsuch lasers which can provide fundamental laser output-power of abouttwo Watts (2.0 W) or greater, and intracavity frequency-convertedarrangements of such lasers which can provide harmonic laseroutput-power of about one-hundred milliwatts (100 mW) or greater.

DISCUSSION OF BACKGROUND ART

The term OPS-lasers, as used herein, refers to a class ofvertical-cavity surface-emitting semiconductor lasers wherein opticalgain is provided by recombination of electrical carriers in very thinlayers, for example, about 150 Ångstrom units (Å) or less, of asemiconductor material. These layers are generally termed quantum-well(QW) layers or active layers.

In an OPS-laser, several QW layers, for example, about fifteen, arespaced apart by separator layers also of a semiconductor material, buthaving a higher conduction band energy that the QW layers. Thiscombination of active layers and separator layers may be defined as thegain-structure of the OPS-laser.

The layers of the gain-structure are epitaxially grown. On thegain-structure is an epitaxially-grown multilayer mirror-structure,often referred to as a Bragg mirror. The combination of mirror-structureand gain-structure is referred to hereinafter as an OPS-structure.

In an (external cavity) OPS-laser, another (conventional) mirror,serving as an output-coupling mirror is spaced-apart from theOPS-structure, thereby forming a resonant cavity with themirror-structure of the OPS-structure. The resonant cavity, accordingly,includes the gain-structure of the OPS-structure. The mirror-structureand gain-structure are arranged such that QW layers of thegain-structure are spaced apart by one half-wavelength of thefundamental laser wavelength, and correspond in position with antinodesof a standing-wave of the fundamental laser-radiation in the resonator.The fundamental-wavelength is characteristic of the composition of theQW layers.

Optical pump-radiation (pump-light) is directed into the gain-structureof the OPS-structure and is absorbed by the separator layers of thegain-structure, thereby generating electrical-carriers. Theelectrical-carriers are trapped in the QW layers of the gain-structureand recombine. Recombination of the electrical-carriers in the QW layersyields electromagnetic radiation of the fundamental-wavelength. Thisradiation circulates in the resonator and is amplified by thegain-structure thereby generating laser-radiation.

OPS-lasers have often been used in the prior art as a means ofconveniently testing QW structures for later use in electrically-pumpedsemiconductor lasers. More recently, OPS-lasers have been investigatedas laser-radiation sources in their own right. The emphasis of suchinvestigation, however, appears to be on providing a compact, evenmonolithic, device in keeping with the generally compact nature ofsemiconductor lasers and packaged arrays thereof.

The gain-structure of OPS-structures may be formed from the same widerange of semiconductor-materials/substrate combinations contemplated fordiode-lasers. These include, but are not limited to, InGaAsP/InPInGaAs/GaAs, AlGaAs/GaAs, InGaAsP/GaAs and InGaN/Al₂O₃, which providerelatively-broad spectra of fundamental-wavelengths in ranges,respectively, of about 960 to 1800 nanometers (nm); 850 to 1100 nm; 700to 850 nm; 620 to 700 nm; and 425 to 550 nm. There is, of course, someoverlap in the ranges. Frequency-multiplication of thesefundamental-wavelengths, to the extent that it is practical, could thusprovide relatively-broad spectra of radiation ranging from theyellow-green portion of the electromagnetic spectrum well into theultraviolet portion.

In conventional solid-state lasers, fundamental-wavelengths, and,accordingly, harmonics thereof (produced by frequency-doubling orfrequency-mixing) are limited to certain fixed wavelengthscharacteristic of a particular dopant in a particular crystalline orglassy host, for example, the well-known 1064 nm wavelength ofneodymium-doped yttrium aluminum garnet (Nd:YAG). While one of thesecharacteristic wavelengths may be adequate for a particular application,it may not be the optimum wavelength for that application.

OPS-lasers provide a means of generating wavelengths, in a true CW modeof operation, which can closely match the optimum wavelength for manylaser applications, in fields such as medicine, optical metrology,optical lithography, and precision laser machining. Prior-artOPS-lasers, however, fall far short of providing adequate power for suchapplications. It is believed that the highest fundamental output-powerthat has been reported, to date, for an OPS-laser is 700 mW at awavelength of about 1000 nm (Kuznetsov, et al., IEEE Photonics Tech.Lett 9, 1063 (1997)). For an intracavity frequency-doubled OPS-laser, itis believed that highest output-power that has been reported is 6 mW ata wavelength of about 488 nm (Alford et al. Technical Digest of theIEEE/OSA Conference on Advanced Solid State Lasers, Boston Mass., Feb.1-3 1999, pp 182-184). It believed that there has been no report to dateof generation of continuous wave (CW) ultraviolet (UV) radiation in anOPS-laser, either directly or by frequency-multiplication.

However flexible an OPS-laser may be in potentially offering a wideselection of wavelengths, in order to be competitive in applications inwhich solid-state and other lasers are currently used, at least anorder-of-magnitude, and preferably two orders-of-magnitude increase inpower over that offered by prior-art OPS-lasers is required. This powerincrease must also be achieved without sacrifice of output-powerstability and beam-quality. Further, in order to be applicable in thebroadest range of applications the range of OPS-laser wavelengthsavailable at high-power and with high beam-quality must be extended intothe UV region of the electromagnetic spectrum, preferably to wavelengthsless than 300 nm.

SUMMARY OF THE INVENTION

The present invention is directed to providing high-power OPS-lasers,including high-power OPS-lasers providing ultraviolet radiation, i.e.,at wavelengths less than about 425 nm. In one particular aspect, anOPS-laser in accordance with the present invention comprises anOPS-structure having a gain-structure surmounting a mirror-structure.The gain-structure includes a plurality of active layers havingpump-light-absorbing layers therebetween. The active layers have acomposition selected to provide emission of electromagnetic radiation ata predetermined fundamental-wavelength between about 425 nanometers and1800 nanometers when optical-pump light is incident on thegain-structure. The mirror-structure includes a plurality of layers ofalternating high and low refractive index and having an opticalthickness of about one-quarter wavelength of the predeterminedwavelength.

A laser-resonator is formed between the mirror-structure of theOPS-structure and a reflector spaced apart therefrom. An opticalarrangement is provided for delivering the pump-light to thegain-structure, thereby causing fundamental laser-radiation having thefundamental wavelength to oscillate in the laser-resonator. A heat-sinkarrangement is provided for cooling the OPS-structure. Anoptically-nonlinear crystal is located in the laser-resonator andarranged for frequency-doubling the fundamental laser-radiation, therebyproviding frequency-doubled radiation having a wavelength half of thefundamental wavelength.

The laser-resonator, the optically nonlinear-crystal, the OPS-structure,the heat-sink arrangement and the optical pump-light-deliveringarrangement are selected and arranged such that the resonator deliversthe frequency-doubled radiation as output-radiation having a wavelengthbetween about 212 nanometers and 900 nanometers at an output-powergreater than about 100 milliwatts. The laser preferably has a resonatorlength greater than about 5.0 cm

In one embodiment of a high-power OPS-laser in accordance with thepresent invention, stable, single axial-mode, CW laser output-power ofabout 4.0 W at 488 nm wavelength is achieved by intracavityfrequency-doubling 976 nm radiation from a single OPS-structure using anoptically-nonlinear crystal of lithium triborate (LBO) in a resonatorhaving a length of about twenty-five centimeters (cm). The OPS-structurehas active layers of an In_(0.18)Ga_(0.82)As composition, andpump-light-absorbing (separator) layers of a GaAs_(0.978)P_(0.022)composition. The laser is pumped by about 34.0 W of 808 nm radiationfrom two diode-laser arrays. Numerical models indicate that the sameresonator may be modified by including an optically-nonlinear crystalfor mixing the fundamental and frequency-doubled radiation to produceabout 120.0 mW of (frequency tripled or third-harmonic) 325 nm UVradiation. A simplified configuration of the resonator of this examplewas used without an optically nonlinear crystal to deliver fundamentaloutput-power of about 10 W.

The 4.0 W of frequency-doubled output-power represents over twoorders-of-magnitude increase over what is believed to be the highestreported frequency-doubled output-power of any prior-art OPS-laser. Itis believed that frequency-tripled output of any power has not beenachieved in a prior-art OPS-laser.

Numerical models indicate that in a resonator similar to the resonatorof the example above, an OPS-structure having a gain-structureIn_(x)Ga_(1−x)P quantum wells with In_(y)Ga_(1−y)As_(z)P_(1−z) separatorlayers therebetween, wherein x is selected to provide fundamentalradiation at 750 nm may be pumped by 12.0 Watts of pump-light at 670 nm,using a 5 mm-long LBO crystal for frequency doubling to provideoutput-power in excess of 1 Watt at the frequency-doubled wavelength of375 nm.

This remarkable increase in OPS-laser output-power and the ability togenerate high, CW, UV output-power, either by frequency-doubling orfrequency-tripling, is achieved without sacrifice of beam-quality.Single mode operation provides that OPS-lasers in accordance with thepresent invention can have a beam quality less than 2.0 times, and aslow as 1.2 times the diffraction limit. This high-beam quality makes theinventive OPS-lasers ideal for applications in which the outputradiation must be focused to a very small spot for making preciseincisions in inorganic or organic material, or must be efficientlycoupled into an optical fiber for transport to a location where it is tobe used.

In another aspect of an OPS-laser in accordance with the presentinvention, the laser includes first and second resonators arranged suchthat a portion of the resonator axes of each are on a coaxial path. Thefirst resonator includes an OPS-structure arranged outside the coaxialpath to provide a selected fundamental-wavelength of laser radiation.Located on the coaxial path of the first and second resonators is anoptically-nonlinear crystal arranged for frequency-doubling thefundamental radiation. The first and second resonators areinterferometrically matched to maintain optimum phase-matching betweenfundamental and frequency-doubled radiation in the optically-nonlinearcrystal. Fundamental-wavelength radiation and frequency-doubledradiation circulate together only along the coaxial path. Anoptically-nonlinear crystal is located in the second resonator outsidethe coaxial path for doubling the frequency of the frequency-doubledradiation thereby providing frequency-quadrupled radiation. Numericalmodels indicate that by using the 976 nm OPS-structure and pumpingarrangement of the above-described first embodiment, this secondembodiment is capable of providing about 2.0 W of frequency-quadrupled(244 nm) radiation.

In yet another aspect of an OPS-laser in accordance with the presentinvention, the laser includes first and secondinterferometrically-matched resonators arranged such that a portion ofthe resonator axes are on a coaxial path. The coaxial path includes afirst optically-nonlinear crystal arranged for frequency-doubling asdiscussed above with respect to the second embodiment.Fundamental-wavelength radiation and frequency-doubled radiationcirculate together only along the coaxial path. A secondoptically-nonlinear crystal is located in the coaxial path of the firstand second resonators for mixing the fundamental radiation withfrequency-doubled radiation thereby providing frequency-tripledradiation. Numerical models indicate that by using the 976 nmOPS-structure and pumping arrangement of the above-described firstembodiment, this third embodiment is capable of providing about 4.0 W offrequency-tripled (325 nm) radiation.

Other, more general, aspects of OPS-lasers in accordance with thepresent invention include but are not limited to: design of heat-sinkconfigurations and bonding methods for cooling an OPS-structure whichallow the above exemplified high pump-powers to be directed on theOPS-structure while maintaining a safe operating temperature therefor;design of optically-long resonators for providing a relatively largefundamental mode-size at OPS-structure to take advantage of a largerpumped-area, thereby increasing laser output-power; design of specific,folded-resonator configurations for optimizing output offrequency-converted radiation and preventing reflection of thefrequency-converted radiation back into the OPS-structure where it wouldbe lost through absorption; selection of a specific ratio of pumped areato mode-size at the OPS to optimize use of gain, and to preventgeneration of transverse modes of oscillation; use of an intracavitywavelength-selective element for preventing oscillation of fundamentalradiation at wavelengths outside the spectral range of acceptance ofoptically nonlinear crystals; selection of optically nonlinear materialsfor maximum spectral acceptance to allow the use of efficient andtolerant wavelength-selective devices for the former; configuration ofOPS-structures to eliminate parasitic lateral oscillation which wouldotherwise reduce output power; design of OPS-structures for minimum netstress and reliability under high power operation; use of radial-indexgradient lens to optimize multiple optical-fiber delivery of pump-light;and design of mirror-structures for the inventive OPS-structure formaximum thermal-conductivity thereby facilitating cooling of theOPS-structures.

It will be particularly evident from the detailed description of thepresent invention presented below that for achieving the high powersdiscussed above, OPS-laser resonators in accordance with the presentinvention depart radically from the “compactness” philosophy ofprior-art OPS-lasers and are inventively configured for intracavityfrequency multiplication. It will also be evident that significantattention is directed to thermal management of OPS-structures, to thedesign of OPS-structures themselves, and to selection of frequencymultiplication materials, in order to achieve the remarkableoutput-power levels, and stability of the inventive OPS-lasers. It willfurther be evident that certain inventive aspects of the invention areapplicable to both high and low-power OPS-lasers, or even to laser typesother than OPS-lasers.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, schematically illustrate a preferredembodiment of the present invention, and together with the generaldescription given above and the detailed description of the preferredembodiment given below, serve to explain the principles of theinvention.

FIG. 1 schematically illustrates a preferred embodiment of an OPS-laserin accordance with the present invention having a resonator including anOPS-structure and an optically-nonlinear crystal arranged forintracavity frequency-doubling the fundamental-wavelength of theOPS-structure.

FIG. 2 is a graphical representation of the mode-size of fundamentalradiation as function of axial position in the resonator of FIG. 1

FIG. 3 schematically illustrates details of one preferred arrangementand composition of semiconductor layers in the OPS-structure of FIG. 1.

FIG. 4 schematically illustrates another preferred embodiment of anOPS-laser in accordance with the present invention having a resonatorincluding an OPS-structure and two optically-nonlinear crystals arrangedfor intracavity frequency-tripling the fundamental-wavelength of theOPS-structure.

FIG. 5 schematically illustrates yet another preferred embodiment of anOPS-laser in accordance with the present invention having a firstresonator including an OPS-structure and a second resonator having acommon optical path with the first resonator, the common optical path ofthe resonators including two optically-nonlinear crystals arranged forintracavity frequency-tripling the fundamental-wavelength of theOPS-structure.

FIG. 6 schematically illustrates still another preferred embodiment ofan OPS-laser in accordance with the present invention having a firstresonator including an OPS-structure and a second resonator having acommon optical path with the first resonator, the resonators beingarranged for intracavity frequency-quadrupling thefundamental-wavelength of the OPS-structure.

FIG. 7 schematically illustrates a further preferred embodiment of anOPS-laser in accordance with the present invention having a straightresonator including an OPS-structure and a birefringent filter.

FIG. 8 schematically illustrates the OPS-structure of any of FIGS. 1 and4-7 bonded to a heat-sink assembly in accordance with the presentinvention.

FIG. 9 is a graph depicting isothermal contours produced by uniformheat-flow into a selected area of a surface of a relatively-massivecopper heat-sink.

FIG. 10 is a graph depicting isothermal contours produced by uniformheat-flow into the selected area of FIG. 7 on a surface of a CVD-diamondlayer bonded to a relatively-massive copper heat-sink.

FIG. 11 is a perspective view schematically illustrating parasiticlateral oscillation in an OPS-structure in accordance with the presentinvention.

FIG. 12 is a graph schematically illustrating computed conversionefficiency as a function of wavelength for LBO and potassium niobate(KNbO₃) crystals of equal-length.

FIG. 13 schematically illustrates a further preferred embodiment of anOPS-laser in accordance with the present invention having a resonatorincluding two OPS-structures and an optically-nonlinear crystal arrangedfor intracavity frequency-doubling the fundamental-wavelength of theOPS-structure.

FIG. 14 schematically illustrates a further preferred embodiment of anOPS-laser in accordance with the present invention having a foldedresonator terminated by first and second external mirrors and beingfolded by a mirror-structure of an OPS-structure.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to the drawings, wherein like components are designated bylike reference numerals. FIG. 1 schematically illustrates an OPS-laser20 in accordance with the present invention. Laser 20 includes aresonator 22 having a longitudinal axis 24 thereof folded by afold-mirror 26. Resonator 22 is terminated at one end thereof by a flatmirror or reflector 28, and at the other end thereof by a mirror portion(mirror-structure) 30 of an OPS-structure 32. A gain portion(gain-structure) 34 of OPS-structure 32 is thus located in the resonatorin contact with a resonator mirror, i.e., mirror-structure 30.

Gain-structure 34 of OPS-structure 32 is an epitaxially-grown monolithicsemiconductor (surface-emitting) multilayer structure including aplurality of active layers (not shown in FIG. 1) spaced apart bypump-light-absorbing separator-layers (also not shown in FIG. 1). Itshould be noted here that the terminology “spaced apart bypump-light-absorbing separator layers” in the context of thisdescription and the appended claims does not preclude there being otherlayers between the QW layers. Depending on the composition of the QWlayers, one or more other layers may be included for strain-management,carrier-confinement and the like. Any such arrangement is applicable inthe context of the present invention.

In prior-art OPS-lasers, the mirror-structure of the OPS-structure istypically also an epitaxially-grown multilayer structure grown fromdifferent compositions of material having a general compositionAl_(x)Ga_((1−x))As (abbreviated AlGaAs), wherein increasing x increasesthe bandgap of the material, and lowers the refractive index. While thisis one form of mirror-structure which could be used for mirror-structure30, it is neither the only form contemplated, nor the preferred formcontemplated. Mirror-structure 30 in an OPS-structure 32 in accordancewith the present invention, need not be epitaxially grown and mayinclude dielectric or metal layers. If epitaxially grown, it need not beformed entirely from materials in the (ternary) AlGaAs system. Preferredmirror-structures are discussed in detail further hereinbelow.

Continuing now with reference to FIG. 1, OPS-structure 32 is bonded inthermal contact with a heat-sink 36. Heat-sink 36 is preferably anactively-cooled heat-sink such as a microchannel-cooler. A particularlypreferred arrangement of heat sink 36 is discussed in detail furtherhereinbelow.

OPS-structure 32 is optically pumped, preferably, by pump-lightdelivered from one or more diode-laser arrays (not shown). In FIG. 1,pump-light is delivered from two diode-laser arrays via two opticalfibers (or fiber bundles) 40. Pump-light 42 diverges as it exits a fiber40. In each case, the diverging pump-light is directed by a mirror 44,through focusing lenses 46 and 48, to be focused (only an axial rayshown) on gain-structure 34 of OPS-structure 32. It should be noted,that while two pump-light-delivery fibers 40 and associated focussingoptics are illustrated in FIG. 1, this should not be considered aslimiting the present invention. Only one, or more than twopump-light-delivery fibers and associated focusing-optics may be used,and even different pump-light sources with or without fiber delivery maybe used, without departing from the spirit and scope of the presentinvention. Further it should be noted that optical fibers and fiberbundles are just one preferred means of transporting pump-light from asource thereof. Other forms of what may be generally termed “alightguide”, for example, solid or hollow light-waveguides maybe usedwithout departing from the spirit and scope of the present invention.

Mirrors 26 and 28, and mirror-structure 30 of OPS-structure 32, eachhave maximum reflectivity at a fundamental (emission) wavelengthcharacteristic of the composition of (active layers of) gain-structure34 of OPS-structure 32. Energizing gain-structure 34 of OPS-structure 32causes laser radiation having the fundamental-wavelength(fundamental-radiation) to circulate in resonator 22. Thisfundamental-radiation is indicated in FIG. 1 by single arrows F.

Included in resonator 22, in folded portion 22A thereof, proximate, butspaced apart from mirror 28, is an optically-nonlinear crystal 50arranged for frequency-doubling (halving the wavelength of) thefundamental radiation. This generates frequency-doubled orsecond-harmonic (2H) radiation indicated in FIG. 1 by double arrows 2H.2H-radiation is generated both on a first pass of fundamental laserradiation therethrough and on a return pass of the fundamental-radiationafter it is reflected from mirror 28. Fold-mirror 26 is transparent tothe 2H-radiation and, accordingly, serves to couple the 2H-radiation outof resonator 22.

It should be noted here, that this folded-resonator arrangement of anOPS-laser in accordance with the present invention is particularlyimportant, considering the high-power operation of the device. Thefolded-resonator arrangement allows formation a resonating beam inresonator 22 having optimum characteristics at OPS-structure 32 and atthe optically-nonlinear crystal 50. In one arrangement the pump-lightspot-size at OPS-structure 32 preferably has a gaussian shape,preferably with a 1/e² radius of about 230 micrometers (μm). In order tomaximize overlap and obtain optimum power-extraction in fundamentaltransverse mode, the resonating fundamental-radiation at OPS-structure32 preferably has a similar size of 230 μm (1/e² radius). Extensivenumerical simulations, corroborated by experimental results, indicatethat the spot-size of fundamental-radiation in optically-nonlinearcrystal 50 is preferably of the order of 50 μm (1/e² radius) for optimumsecond harmonic generation. FIG. 2 depicts mode-size (radius) as afunction of axial position within resonator 22 of laser 20 of FIG. 1. Itcan be seen that this preferred resonator arrangement of plane endmirrors (mirror-structure 30 and external mirror 28) folded by a concavefold-mirror 26 (here, having a radius of 100 mm), providesabove-discussed preferred spot or mode-sizes at OPS-structure 32 and inoptically-nonlinear crystal 50. The folded-resonator arrangement, asnoted above, also allows two passes of fundamental radiation throughoptically-nonlinear crystal 50, thereby increasing the amount of2H-radiation generated. Extraction of the 2H-radiation via fold-mirror36 prevents 2H-radiation from being lost by absorption in OPS-structure32.

In an intracavity frequency-doubled laser in accordance with the presentinvention, it is preferable to include a wavelength-selective element,such as a birefringent filter or an etalon in the resonator for forcingthe resonator to oscillate precisely at the wavelength that theoptically-nonlinear crystal is arranged to frequency-double. This isdiscussed in detail further hereinbelow with respect to selection of apreferred material for optically-nonlinear crystal 50. In laser 20, sucha wavelength-selective element is depicted in the form of a birefringentfilter 52 arranged at Brewster's angle (here 57.1°) to axis 24 ofresonator 20. It is emphasized here that the purpose of thiswavelength-selective element is not axial-mode selection, as this isaccomplished by a combination of the unique properties of OPS-structure32 combined with its location in resonator 22. Rather, birefringentfilter 52 is used to effectively spectrally narrow the gain-bandwidth ofgain-structure 34 of OPS-structure 32 to a bandwidth narrower than aspectral acceptance region over which the optically-nonlinear crystal 50is effective. This prevents laser 20 from oscillating at wavelengthswhere the optically-nonlinear crystal is ineffective. This aspect ofOPS-lasers in accordance with the present invention is also discussed indetail further hereinbelow.

In one example of a laser 20 in accordance with the arrangement of FIG.1, OPS-structure 32 (see FIG. 3) has a gain-structure 34 comprisingfifteen QW or active-layers of an In_(0.18)Ga_(0.82)As composition,having a thickness of about 75.0 Angstrom Units (Å) providing a nominalfundamental (emission) wavelength of 976 nm. Between the QW layers arepump-light-absorbing (separator) layers of a GaAs_(0.978)P_(0.022)composition having a thickness of 1217 Å. Between the QW layers and theseparator layers is a strain-relieving layer of GaAs having a thicknessof about 50 Å. Mirror-structure 30 comprises 27 pairs or periods ofalternating layers of GaAs having a refractive index of about 3.51 andAlAs_(0.96)P_(0.04) having a refractive index of about 2.94 and anoptical thickness of one-quarter wavelength at thefundamental-wavelength. Gain-structure 34 also includes acarrier-confinement layer of Ga_(0.51)In_(0.49)P, having a thickness of1588 Å, between the last separator layer and mirror-structure 30. At anopposite extremity of gain-structure 34 there is also acarrier-confinement layer of Ga_(0.51)In_(0.49)P having a thickness of1588 Å.

OPS-structure 32 is epitaxially grown on an n-type GaAs wafer(substrate), gain-structure 34 being grown first, beginning with thecarrier confinement layer. Mirror-structure 30 is epitaxially-grown onthe gain-structure. After the OPS-structure is grown, the wafer isetched away. The first-grown confinement layer serves as an etch-stoplayer when the substrate is removed by etching. The wafer, andstructures grown thereon, is diced into several OPS-structures 32 in theform of square “chips” about 2.0 mm by 2.0 mm.

An OPS-structure (chip) is first bonded to a microchannel-cooler (cooler36). One preferred microchannel-cooler is a Model SA-2, available fromSaddleback Aerospace Corporation of Los Alamitos, Calif. Before bondingthe OPS-structure to the microchannel-cooler, a relatively thin (about0.3 mm thick) synthetic diamond (CVD-diamond) layer, is bonded to themicrochannel-cooler. The synthetic diamond layer has a particularthermal-management function which is discussed in further detailhereinbelow.

In prior-art methods for bonding to diamond a gold overcoated platinummetalization is applied to the diamond before bonding. Indium is solderused for bonding. Under the operating conditions contemplated for abonded OPS-structure in accordance with the present invention, the goldovercoating can dissolve in the indium bonding material, eventuallycausing long-term instability of the bonded joint, with an associateddeformation of the OPS structure. One preferred bonding method inaccordance with the present invention which is believed may providesuperior long-term stability and avoid potential deformation is toprovide metalization on the diamond comprising a titanium layerovercoated by a platinum layer only without a gold overcoating. Bondingis then performed using indium solder. It is believed that such abonding method would be superior to prior-art chip-bonding methods, notonly under the high-power operating conditions contemplated forOPS-lasers in accordance with the present invention but for anyOPS-laser.

Another possible bonding method would be to use a more stable, gold-tineutectic solder rather than indium solder. It is possible, here,however, that a coefficient of thermal expansion (CTE) mismatch withdiamond may limit its use with diamond heat-sinks. The CTE mismatchproblem may be avoided by use of alternate heat-sink materials having acloser CTE match to the gold-tin eutectic, such as copper-tungsten(Cu—W), cubic boron nitride, silicon-diamond composite, and the like.Some compromise in output-power performance may be expected here,however, due to the lesser heat conduction efficiency of these materialscompared to diamond. The CTE mismatch problem becomes greater the largerthe chip.

After the OPS-structure is bonded to thediamond-layer/microchannel-cooler, the GaAs substrate is removed byetching. Preferably, an antireflection coating is deposited onthus-exposed gain-structure 34 to improve entry of pump-light into thegain-structure.

Regarding optical pumping of OPS-structure 32, each fiber 40 deliversabout 17.0 W of 795 nm radiation from a FAP-30C-800-B diode-laser-arraypackage available from Coherent Semiconductor Group of Santa Clara,Calif. Mirrors 44 are dielectric-coated mirrors having greater than99.9% reflectivity at 795 nm and 28° angle of incidence. Lenses 46 arecemented doublets having a focal length of 40.0 mm and a diameter of18.0 mm. Lenses 48 are cemented doublets having a focal length of 21.0mm and a diameter of 14 mm. These lenses are available from Melles Griotof Irvine, Calif. The pump-light is focused by the mirrors and lensesinto an area of OPS-structure. A total of 34 Watts of pump-light in thepumped area has a substantially Gaussian intensity profile with a radiusof about 260 μm at the 1/e² points. The exemplified diode-laser-arraypackages providing the pump-light require about 50.0 Watts each ofso-called “wallplug” electrical-power input generate 20.0 Watts of pumpradiation coupled into transport fibers 40.

Birefringent filter 52 is quartz plate having a thickness of 3.08 mm andoriented as depicted in FIG. 1 at 57.1 degrees to axis 24, with thequartz optical axis in the plane of the plate. Such a filter isavailable as Part No. BF254-6T from VLOC Company of Port Richey, Fla.This orientation of birefringent filter 52 provides that fundamentalradiation F is polarized perpendicular to the plane of FIG. 1 asillustrated by arrow P1.

Birefringent filter 52 has narrow transmission-peaks separated by about35 nm, each with a full width at half maximum transmission (FWHM) ofabout 3 nm. Maximum selectivity is achieved by keeping the quartzoptic-axis at an angle of about 45 degrees from an axis defined by theintersection of the vertical plane with the plane of the plate. Thewavelength of the transmission peaks can be shifted by rotating theplate slightly around an axis normal to its faces, thus achieving tuningof the filter. The computed tuning rate, confirmed experimentally, isabout 5.6 nm per degree of rotation

Mirror 28 is a plane mirror and fold-mirror 26 is a concave mirrorhaving a radius of curvature of 100 mm. Mirrors 26 and 30 are axiallyseparated by a distance of 202 mm. Mirrors 26 and 28 are axiallyseparated by a distance of 56 mm. Accordingly, resonator 22 has an axiallength of 258 mm (25.8 cm).

Optically nonlinear crystal 50 is a 5 mm-long crystal of lithiumtriborate (LBO) having a cross-section of 3 mm×3 mm. The crystal is cutfor type-1 phase matching for 976 nm radiation. Propagation of thefundamental beam is in the crystallographic X-Y plane. The propagationdirection is at an angle of 17.1 degrees from the X-axis. Thefundamental-radiation is polarized perpendicular to the X-Y plane(parallel to the Z-axis). Second-harmonic radiation is polarized in theX-Y plane, as illustrated by arrow P2.

The above-specified exemplary OPS-laser generated an intracavity powerof fundamental (976 nm) radiation of about 300 watts yieldingfrequency-doubled (488 nm) radiation at an output power of 5 Watts insingle longitudinal (axial) mode and single transverse mode. The beamdivergence was measured at 1.2 times the diffraction limit (M²=1.2). Thequantity M² is a numerical measure which represents a ratio of the sizeof the beam to a diffraction-limited size. A high quality beam may beregarded as a beam having an M² of about 2.0 or less. The high-beamquality available with the inventive OPS-lasers make them useful forapplications in which the laser output-radiation must be focused to avery small spot for making precise incisions in inorganic or organicmaterial, or must be efficiently coupled into an optical-fiber or guidedby an articulated arm for transport to a location where it is to beused.

From the power point-of-view alone, this represents about aone-thousand-fold increase over the above-discussed 6.0 mW output of aprior-art IC, frequency-doubled OPS-laser. Surprisingly, the fundamentalradiation, and, accordingly, the frequency-doubled radiation was in asingle axial (longitudinal) mode. Temporal stability of theoutput-radiation is estimated in terms of output-fluctuation or noise ofless than about 0.05% RMS over a band DC to 10 MHz. It is believed thatin other embodiments OPS-lasers in accordance with the present inventiondescribed hereinbelow a noise of less than about 0.1% can be obtained.Pump-light to second-harmonic efficiency is about ten percent.Electrical to second-harmonic efficiency is about four percent.Generally, a pump-light to second-harmonic efficiency of 3.0 percent orgreater may be achieved in appropriate embodiments of inventiveOPS-lasers discussed herein.

Detuning of the cavity (moving mirror 28 axially away from fold-mirror36) to reduce the beam spot-size at OPS-structure 32, forced operationin multiple transverse mode, allowed obtaining up to 7.5 Watts ofoutput-power at 488 nm, thereby obtaining even higher power andefficiency at the expense of a reduced beam-quality due to the multiplemode operation. The relationship of mode-size to pumped-area (pump-spotsize) in connection with ensuring single axial-mode operation isdiscussed in detail further hereinbelow.

The high output-power at 488 nm of the inventive IC double OPS-laser isobtained with a total input electrical-power to the pump-light-providingdiodes of about 100 Watts. In prior-art lasers, multi-Watt,high-beam-quality radiation at 488 nm was only obtainable as the outputof argon ion (gas) lasers. These lasers typically require severalthousands of Watts (up to 30 Kilowatts) of electrical-power input.Accordingly, the above-exemplified laser in accordance with the presentinvention provides an improvement in the “electrical-to-optical”efficiency of two orders of magnitude for 488 nm lasers.

Continuing now with the description of preferred embodiments ofOPS-lasers in accordance with the present invention, an important aspectof the above-discussed example of an OPS-laser is the discovery thatsuch a high-fundamental power can be generated from an OPS-structure, bypaying attention, inter alia, to thermal management of theintensely-pumped OPS-structure in combination with increasing thedimensions of the resonator well beyond those of prior-art OPS-lasers.

Having established experimentally that such a high fundamental and2H-power can be provided, it is possible to numerically-model otherinventive OPS-lasers based on the same or different OPS-structures 32and “long resonator” arrangements, similar to that of laser 20, whichgenerate other fundamental and 2H wavelengths at similar output power.

In particular, high power CW radiation in the ultraviolet spectralregion, for example, at about 375 nm, can be generated by using anOPS-structure 32 having a gain-structure 34 having In_(x)Ga_(1−x)Pquantum wells with In_(y)Ga_(1−y)As_(z)P_(1−z) separator layerstherebetween. Such an OPS-structure, pumped with 670 nm radiation fromfiber-coupled diode-lasers (for example, SDL part number 7470 P-5 fromSDL, Inc., of San Jose, Calif.) provides gain at 750 nm. A resonatorsimilar to resonator 22 of FIG. 1 may be used, with a similarbirefringent filter 52, suitably angle-tuned, and with anoptically-nonlinear crystal 50 of LBO arranged for frequency doublingthe 750 nm radiation. In this regard, the LBO crystal is preferably cutfor propagation in the X-Y crystallographic plane, at an angle of 37.5degrees from the X-axis. The polarizations of the fundamental and2H-radiation are the same as for the (976 nm fundamental-wavelength)laser 20 exemplified above.

The efficiency of conversion for 750 nm radiation for the LBO crystal iscomputed to be slightly higher (17% higher) than the efficiency ofconversion for 976 nm radiation for equal lengths of theoptically-nonlinear crystal. This results from a balancing of twoopposing factors. The effective nonlinearity (nonlinear-coefficient) ofLBO at 750 nm is lower, at about 1.0 picometer per volt (pm/V) comparedwith a value of 1.2 pm/V at 975 nm, due to the different angle ofpropagation. The overall conversion-efficiency, however, is proportionalto the square of the nonlinear-coefficient, but is also proportional tothe inverse of the square of the wavelength. Accordingly, the shorterwavelength (750 nm), is instrumental in providing the above-discussedimprovement of the overall conversion-efficiency.

In one numerically-simulated example of a frequency-doubled, 750 nmfundamental-wavelength laser 20, 12.0 Watts of pump-light at 670 nm,obtained from 4 fiber-coupled diode-laser arrays or modules, using a 5mm-long LBO crystal for optically-nonlinear crystal 20, resulted incomputed output-power in excess of 1 Watt at the 2H-wavelength of 375nm. Using higher pump-power from more the same number of more-powerfuldiode-laser arrays, or from more arrays of the same power, can provide375 nm output-power comparable to the 488 nm output power exemplifiedabove for the same resonator. Use of more pump-light arrays is possibleusing an alternative pump-light delivery arrangement discussed in detailfurther hereinbelow.

Having established experimentally, and numerically corroborated, thatsuch a high fundamental power can be generated, it is possible toconfigure other “long-resonator” type, inventive OPS-lasers based on thesame OPS-structures, for generating third harmonic (3H) and fourthharmonic (4H) radiation, and to compute, with confidence, whatoutput-power can be generated at such third and fourth harmonicwavelengths in the UV region of the electromagnetic spectrum.Descriptions of such resonator configurations are set forth below,exemplifying generation of the third harmonic of 750 nm radiation, andthe fourth harmonic of 976 nm radiation.

Referring now to FIG. 4, a laser 60 including a resonator 23 isschematically illustrated. Laser 60 is identical in most regards tolaser 20 of FIG. 1, with the following exceptions. Mirror 28 is coatedfor high reflectivity at the fundamental and second (2H) harmonicwavelengths. Mirror 26 is maximally reflective at thefundamental-wavelength and highly transmissive at the 2H and3H-wavelengths. An optically-nonlinear crystal 62, is located betweenoptically-nonlinear crystal 50 and mirror 26 in folded arm 23A ofresonator 23. Optically-nonlinear crystal 62 is arranged to mix thefundamental and 2H wavelengths (frequencies), thereby generatingfrequency-tripled radiation, indicated in FIG. 4 by triple arrows 3H.Both 2H and 3H-radiation leave resonator 23 via fold-mirror 26. The 2Hand 3H-radiations are then separated by a dichroic beamsplitter 64.

Now, third-harmonic generation (mixing of fundamental and 2Hfrequencies) is known to be proportional (dependent onoptically-nonlinear crystal characteristics) to the product of thefundamental and second-harmonic power. So, based on experimentallyestablished values for these powers in this resonator, and on documentedvalues for 3H-generation efficiency of various optically-nonlinearmaterials, it can be predicted with a high degree of confidence that theabove-described example of a laser 60 using β-barium borate (BBO) foroptically-nonlinear crystal 62 can generate about 150 mW of true-CW,single-mode, laser output-power at a wavelength of 250 nm, assuming anintracavity fundamental power of about 300 W at 750 nm generating about5.0 W of 2H-radiation (375 nm).

A BBO crystal (crystal 62) suitable for this process is cut forpropagation at an angle of 48 degrees from the optic axis, in a planecontaining the optic (Z) axis and making an angle of 30 degrees with theX-Z crystallographic plane. Fundamental-radiation and 2H-radiationpropagate with polarization perpendicular to the optic axis, (ordinarypolarization). The 3H-radiation is generated with extraordinarypolarization (normal to the fundamental polarization).

Computations based on published data from crystal manufacturers (CASIX)indicate that by focussing 300 watts of 750 nm radiation down to aradius of 60 μm in optically-nonlinear crystal 62 (in arm 20A ofresonator 20), together with 5 Watts of 2H-radiation (375 nm) focusedcoaxially in the same crystal to a radius of 50 nm, about 150 mW of 250nm (3H) radiation is generated. The above-described BBO crystal employstype-I phase matching. This means that the fundamental and the 2Hradiation must have the same polarization within the crystal.

The second-harmonic-generating LBO crystal (optically-nonlinear crystal50), however, generates 2H-radiation with polarization orthogonal to thepolarization of the fundamental-radiation. The polarization of the2H-radiation, accordingly, must be rotated by 90 degrees before itenters optically-nonlinear crystal 62. This may be achieved by abirefringent quartz plate (polarization rotator) of such design that ithas retardation of an even integer multiple of π for the fundamentalradiation, and retardation of an odd integer multiple of π for the 2Hradiation.

Such a polarization-rotator (depicted in FIG. 4 in phantom aspolarization-rotator 65) is inserted in the common path (arm 20A) of thefundamental and 2H-radiation between optically nonlinear crystals 50 and62. Polarization-rotator 65, leaves the polarization of thefundamental-radiation unaltered, and rotates the linear polarization ofthe 2H-radiation by an amount equal to twice the angle between theinitial polarization and the direction of the optic-axis of theretardation plate.

As an alternative, a BBO crystal may be cut for type-II mixing, so thatpolarization-rotator 65 is not required to modify the polarization ofthe 2H-radiation. Conversion in this case, however, can be expected tobe less efficient.

Providing a resonator mirror or reflector on any optically nonlinearcrystal in a OPS-laser resonator in accordance with the presentinvention is not precluded. However, additional measures such astemperature control of the crystal may be required to resolve theconflicting alignment requirements for the resonator and the cyrstal.

In resonator 23 of FIG. 4, third-harmonic generation from 970 nmfundamental-radiation is also possible using the alternatives discussedabove for 750 nm fundamental radiation. Numerical simulations indicatethat such third-harmonic generation could generate about 150 mW of 325nm radiation. Those skilled in the art, from the above discussed examplecan determine appropriate nonlinear crystal parameters without furtherdetailed description. Accordingly, no such detailed description ispresented herein.

The 150 mW of third-harmonic radiation exemplified above represents morethan a twenty-times increase in power on third-harmonic generation overthe maximum believed to have been reported in the prior art for onlysecond-harmonic generation. The arrangement for generating this thirdharmonic radiation represented by laser 60, however, is arguably asomewhat inefficient approach to IC generation of third harmonicradiation. Significant improvements in harmonic output-power arepossible in another family of OPS-laser resonators in accordance for thepresent invention discussed below, beginning with reference to FIG. 5

In FIG. 5, yet another embodiment 70 of an OPS-laser in accordance withthe present invention is schematically depicted. Laser 70 includes tworesonators 25 and 27 having a common, axial path 35 (indicated in boldline in FIG. 5) over a portion of their total length. Resonator 25 maybe defined as a “fundamental resonator” and is terminated at one endthereof by mirror 28 and at the other end thereof by mirror-structure 30of OPS-structure 32. Resonator 25 is folded by a fold-mirror 26.OPS-structure 32 is mounted and pumped as described above for lasers 20,and 60. Only one pump-light-delivery fiber and associated focussingoptics are depicted in FIG. 5, for clarity.

Resonator 27 may be defined as a “second-harmonic resonator”. It is usedto build up 2H-radiation to levels much higher than are achieved in thesimple double-pass arrangements of above-described resonators 22 and 23.Resonator 27 is terminated at one end thereof by mirror 28 and at theother end thereof by a mirror 74. Resonator 27 is also folded by afold-mirror 26. Fold-mirror 26 in this embodiment is coated for maximumreflection at the fundamental and 2H-wavelengths and maximumtransmission at the 3H-wavelength. Mirror 28 in this embodiment iscoated for maximum reflection at the fundamental, 2H and 3H-wavelengths.The axes of the resonators are folded together by a dichroicbeamsplitter 72 coated for maximum reflection at the 2H-wavelength andmaximum transmission at the fundamental-wavelength. Accordingly, thecommon (here, folded) axial path of resonators 25 and 27 extends betweenbeamsplitter 72 and mirror 28. It is pointed out here, that all coatingsmentioned above are believed to be within the capabilities of commercialsuppliers of optical coating services. One such supplier is CoherentAuburn Group, of Auburn, Calif.

Located in the common axial-path of resonators 27 and 27, proximatebeamsplitter 72, is an optically-nonlinear crystal 50 arranged fordoubling the fundamental-wavelength. The 2H-radiation generated by thefrequency-doubling circulates and builds up in resonator 27, therebygreatly increasing the amount of intracavity 2H-radiation compared withthe simple “double-pass” arrangement of laser 20 of FIG. 1. Located incommon, folded portion 27A of resonators 25 and 27, (wherein, of course,both fundamental and 2H-radiation are circulating) is anoptically-nonlinear crystal 62 arranged for mixing the fundamental and2H-radiation to generate 3H-radiation. 3H-radiation so generated escapesthe resonators via fold-mirror 26. Regarding the resonators in general,the following should be noted.

It is preferable that resonators 25 and 27 have about the same lengthand are similarly optically configured. This provides that the mode-sizefor fundamental and 2H-radiation is about the same, thereby maximizingmixing efficiency. It is also preferable that the resonators areinterferometrically-matched to maintain an optimum phase-relationshipbetween the circulating fundamental and 2H-radiation at the location ofoptically-nonlinear crystal 50. This is required to ensure that2H-radiation generated in optically-nonlinear crystal 50 is added, inphase, to the 2H-radiation circulating in the 2H-resonator. This ensuresthat circulating 2H-radiation can grow to high levels, thereby providinga high conversion-efficiency in the BBO mixing-crystal (opticallynonlinear crystal 62). In this regard, it is also preferable thatfundamental resonator 25 operate in only a single axial-mode to avoidloss of power from mode-competition. Conditions for providing singleaxial-mode operation are discussed in detail further hereinbelow.

One arrangement for providing interferometric-matching of the resonatorsis to provide an axial driver 76, such as a piezo-electric driver, formirror 74 of resonator 27. As mirror 74 is axially-driven, circulating2H-radiation rises to a maximum level and falls to a minimum level(which may be zero or near-zero) corresponding respectively to amaximally-in-phase condition and a maximally-out-of-phase condition atoptically-nonlinear crystal 50 for the fundamental and 2H-radiation(standing waves). Correspondingly, 3H-output-power would rise and fall.A beamsplitter 78 directs a small sample-fraction 3HS of the 3H-outputto a detector 80. Detector 80 is cooperative with a controller 82 whichadjusts driver 76 to maintain peak output-power.

As noted above, third-harmonic generation is proportional to the productof the fundamental and second-harmonic power. By incorporating a secondresonator for 2H-power, laser 70 provides that a much greater 2H-poweris available for mixing than is the case in laser 60 of FIG. 4, and,further, without any increase being required in the fundamental-power.So, based on above-discussed values for available fundamental-powers inthis resonator and documented values for 2H-generation efficiency of LBOit can be numerically determined that 300 W of circulating 750 nmradiation would generate about 200 W of circulating 375 nm radiation inresonator 27 the double-pass combination of these in an opticallynonlinear crystal 62 of β-barium BBO can generate about 4 W of true-CW,single-mode, laser output-power at a wavelength of 250 nm. Similaroutput-power levels for 325 nm radiation can be determined for3H-conversion of 976 nm radiation. This is a more than asix-hundred-times increase in power on third-harmonic generation overthe maximum believed to have been reported for only second harmonicgeneration in prior-art OPS-lasers.

An optically-nonlinear (mixing) crystal preferable for laser 70 is thesame as that discussed above with reference to laser 60. Care should beexercised in the design of resonator 25 of laser 70, however, inparticular concerning the sizes of the beams within theoptically-nonlinear crystals 50 and 62.

The fundamental and 2H-beams are preferably focused tightly in opticallynonlinear crystal 62 to maximize conversion (mixing) efficiency. Anoptimum range of larger beam sizes, however, is preferred forfundamental-radiation in optically-nonlinear (doubling) crystal 50.Extensive numerical modeling of coupled resonators 25 and 27 indicatesthat for an LBO crystal having a length of 15 mm, the spot-sizes of thefundamental beam at the OPS-structure and in the doubler crystal arepreferably about the same. This result simplifies the design of theresonator, since it makes it possible to place the doubling crystal inthe proximity of the OPS-structure (as illustrated in FIG. 5), withouthaving to create, in the common optical path of resonators 25 and 27, atightly-focused waist in which to place optically-nonlinear (doubling)crystal 50. In one preferred resonator configuration, the fundamentalbeam has a spot-size of about 200 μm at OPS-structure 32, a spot-size ofabout 200 μm in optically-nonlinear crystal 50, and a spot-size of about50 μm in optically-nonlinear crystal 62. The 2H-resonator 27 is designedso that the spot-size of the 2H-beam in optically-nonlinear crystal 50is about 150 μm, and in optically-nonlinear crystal 62 is about 40 μm.

Referring now to FIG. 6, still another embodiment 90 of an OPS-laser inaccordance with the present invention is depicted. Laser 90 includes tworesonators 29 and 31 having a common axial path 37 (indicated in boldline in FIG. 4) over a portion of their total length. Resonator 29 maybe defined as a “fundamental resonator” and is terminated at one endthereof by mirror 26 and at the other end thereof by mirror-structure 30of OPS-structure 32. OPS-structure 32 is mounted and pumped as describedabove for lasers 20, and 60. Only one pump-light-delivery fiber andassociated focussing optics are depicted in FIG. 6, for clarity.

Resonator 31 may be defined as a “second harmonic resonator”. Resonator31 is terminated at one end thereof by mirror 74, and at the other endthereof by a mirror 92. Resonator 31 is folded by a fold-mirror 94.Mirror 28 in this embodiment is coated for maximum reflection at thefundamental-wavelength. Mirror 74 is coated for maximum reflection atthe 2H-wavelength. Mirror 92 is coated for maximum reflection at thefundamental, 2H, and, fourth harmonic (4H) wavelengths. Mirror 94 iscoated for maximum reflection at the 2H-wavelength and maximumtransmission at the 4H-wavelength

The axes of the resonators are folded together by dichroic beamsplitters72 and 73, each thereof coated for maximum reflection at the2H-wavelength and maximum transmission at the fundamental-wavelength.Accordingly, the common axial-path of the resonators 29 and 31 extendsonly between beamsplitters 72 and 73. For reasons discussed above,mirror 74 is mounted on a driver 76 controlled by a controller 82 inaccordance with power detected by detector 80 tointerferometrically-match resonators 29 and 31.

Optically-nonlinear crystal 50 is arranged for frequency-doublingfundamental-radiation circulating along common path 37 of resonators 29and 31. Frequency-doubled radiation so generated circulates and buildsup in resonator 31. Another optically-nonlinear crystal 67 is located infolded portion 31A of resonator 31 between mirrors 92 and 94 thereof.Optically-nonlinear crystal 67 is arranged for frequency-doublingcirculating frequency-doubled radiation 2H thereby generatingfrequency-quadrupled (4H) radiation. The 4H-radiation exits resonator 31via fold-mirror 94.

Such a resonator configuration is advantageous in the generation of 244nm radiation through fourth-harmonic generation of 976 nm. A preferredoptically-nonlinear (doubling) crystal 50 for this application is an LBOcrystal having a length of about 15.0 mm, with orientation of thecrystal-axes identical to those described above with reference to laser20 of FIG. 1. Optically-nonlinear (quadrupling) crystal 67 is preferablya BBO crystal having a length of about 8.0 mm. This crystal ispreferably cut for propagation at an angle of 54 degrees from theoptic-axis, in a plane containing the optic (Z) axis and making an angleof 30 degrees with the X-Z crystallographic plane of theoptically-nonlinear crystal.

The 2H-radiation propagates with polarization perpendicular to the opticaxis, (ordinary polarization) the 4H radiation is generated withextraordinary polarization (normal to the 2H-polarization). Preferablymirror 26 has a radius of curvature of 30 cm., mirror 94 has a radius ofcurvature of 10 cm, and mirrors 74 and 92 are flat mirrors.

Similar considerations apply for the beam spot-size inoptically-nonlinear crystals 50 and 67 as apply for crystals 50 and 63of FIG. 5. Numerical simulations indicate that an optimum, relativelylarge, beam-size exists for the beams inside the optically-nonlinear(doubler) crystal 50. The fundamental-radiation spot-size inoptically-nonlinear (doubler) crystal 50 is preferentially the same asthe spot-size at OPS-structure 32, for a crystal length of about 15 mm.The “second harmonic resonator” 31 is dimensioned so as to generate a 2Hspot-size in optically-nonlinear crystal 67 equal to the fundamentalbeam spot-size divided by the square root of two (1.414), so that a beamsustained by the resonator 31 has the same transverse size as beamgenerated by optically-nonlinear crystal 50. The 2H-radiation is focusedby concave fold-mirror 94 into the optically-nonlinear crystal 67, to aspot-size of about 50 microns (1/e² radius).

Another preferred material for optically-nonlinear crystal 67 is cesiumlithium borate (CLBO). This material can advantageously substitute BBOfor frequency-doubling 488 nm radiation in this and other applications.A CLBO crystal for frequency-doubling 488 nm radiation is preferably cutfor propagation of the beam at an angle of 75.7 degrees from the Z-axis,in a plane containing the Z axis and at an angle of 45 degrees from theX-Z plane. The 244 nm radiation so generated is polarized in the planedefined by the direction of propagation and the Z-axis (extraordinarypolarization), the 488 nm radiation is polarized perpendicular to the244 nm radiation. The nonlinear-efficiency of CLBO for this applicationis the same as that of BBO. CLBO, however, has a significant advantageof a five-times-greater angular acceptance than BBO, and a four-timessmaller walk-off angle than BBO. A greater acceptance angle and smallerwalk-off angle both contribute to increasing the netconversion-efficiency of an optically nonlinear crystal. First orderconsiderations of acceptance angle and walk-off angle indicate that theconversion-efficiency of CLBO may potentially be several times higherthan the efficiency of BBO. These first-order considerations areapplicable in general to frequency-doubling radiation having awavelength between about 425 nm and 525 nm in any resonator. Theradiation in that wavelength range may be the fundamental-radiation orharmonic radiation of any gain-medium.

While embodiments of high-power OPS-lasers in accordance with thepresent invention are described above with respect to intracavityfrequency-multiplication arrangements, it will be evident to one skilledin the art that high-power OPS-laser in accordance with the presentationmay be operated, without intracavity optically-nonlinear crystals,simply as an efficient source of high-power single-mode fundamentalradiation. A fundamental output power of at least 2 W and even greaterthan 5 W may be achieved in fundamental resonator embodiments of the anOPS-laser in accordance with the present invention.

By way of example, 10 W of single axial-mode fundamental radiation at976 nm was generated in an OPS-laser 20F (see FIG. 7) including a simplelinear (not folded) resonator 22F formed by a concave output-couplingmirror 28F and mirror structure 30 of OPS-structure 32. Mirror 28F has aradius of curvature of 30 cm and is spaced apart from OPS-structure 32by a distance of about 15.0 cm. A birefringent filter 52 is preferablyincluded in resonator 22F. This allows for output-wavelength adjustmentwhich may be required if manufacturing tolerances of OPS-structure 32provide peak gain at some wavelength slightly displaced from a desiredoutput-wavelength.

Such an inventive OPS-laser could be used as a radiation-source forextracavity frequency-multiplication using either a single-passarrangement of optically-nonlinear crystals or by directing thefundamental-radiation into a prior-art extracavity ring-resonator,including, or formed from, an optically-nonlinear material. A high-powerOPS-laser in accordance with the present invention may also provide asource of fundamental (pump) radiation for an optical parametricoscillator (OPO), co-linearly or non-colinearly pumped. Sucharrangements are disclosed in co-pending U.S. patent application Ser.No. 09/179,022 by the same inventors, the complete disclosure of whichis hereby incorporated by reference.

Proceeding now with a description of mode-control and other scalingaspects of the present invention, a surprising discovery was that singleaxial-mode operation of the inventive lasers was simply achieved even atthe relatively long resonator-lengths required for the exemplifiedhigh-power operation. The long resonator length is required for reasons,inter alia, as follows.

The gain-structure of an OPS has some inherent limitations due to theneed to strongly absorb pump-light in layers separating the QW layers.These QW layers are preferably optically-spaced apart by ahalf-wavelength of the fundamental radiation. Prior-art investigationsof such structures seem to have resulted in a general belief amongpractitioners of the relevant art that a number of about fifteen such QWlayers is about optimum. This results at least from a consideration thatthe pump-light, through absorption in the structure, may not penetrateeffectively more than fifteen half-wavelengths deep. It should be notedhere that use of gain-structures with 15 QW layers in examples ofOPS-lasers in accordance with the present invention should not beconstrued as limiting the inventive OPS-lasers or indicating thatoptimization of the depth and QW content of such gain-structures hasreached a limit. In developing the inventive OPS-lasers, emphasis hassimply been placed, with evident success, at addressing other scalingissues and other structural issues of OPS-structures. A discussion ofsuch other issues is set forth below.

Considering, for the purposes of this description that some limit,whatever it is, to the depth or QW layer content of OPS gain-structuresexists, then another approach to increasing available gain (accordinglypower) in the gain-structure is necessary. One such approach toincreasing output-power in accordance with the present invention is toincrease the pumped-area on the OPS-structure.

In order to take advantage of a larger pumped-area, however, a resonatormust be increased in length such that the resonator mode-size issufficient to extract all of the gain available in the pumped-area.Resonator length is increased in proportion to the square of theincrease in mode size. Accordingly, with increasing power being derived,inter alia, from increasing mode size, the dimensions of the inventiveOPS-resonators radically depart from the prior-art compactness whichappears from prior-art teachings to be an attractive feature ofOPS-lasers. Indeed, dimensions of the inventive OPS-lasers becomecomparable with the dimensions of prior-art diode-pumped solid-state(DPSS) laser-resonators of similar output-power and beam-quality. Aswill be evident from this description, however, these inventiveresonators, particularly those arranged for IC frequency-conversion,have certain characteristic design features particularly related to theuse of an OPS-structure as a gain-medium.

One originally anticipated problem in drastically increasing the lengthof an external OPS-laser resonator is the problem of mode-control,specifically, operating in a single axial-mode. Operation in a singleaxial-mode is preferred, inter alia, for maximizing and decreasingtemporal fluctuation of output-power. As noted above operating a“fundamental resonator” in a single axial-mode is particularly preferredin multiple-resonator IC frequency-multiplication embodiments ofOPS-lasers in accordance with the present invention.

In above-mentioned prior-art DPSS-lasers, as the length of a resonatoris increased, the number of possible axial (longitudinal) modes ofoscillation is also increased, absent any measures for preventing this.It would also be anticipated, on prima facie consideration, that thisproblem would exacerbated in an OPS-laser because of the greatergain-bandwidth of a semiconductor gain-medium compared with asolid-state (dopant:host) gain-medium of a DPSS-laser.

Typically, a large number of axial-modes leads to a fluctuating (noisy)power output as the modes compete chaotically for gain in thegain-medium. Prior-art measures which have been taken in DPSS-lasers tolimit the number of oscillating modes and reduce output-noise includeselective placement of the gain-medium in the resonator impositionswhere phase-relationships between adjacent modes limit thismode-competition. This approach has met with only limited success inprior-art DPSS-lasers. It is believed that, practically, true singlemode operation has only been possible in prior-art lasers bymechanically stabilizing the resonator and including a highlywavelength-selective element in the resonator, with its attendantproblems of resonator loss, maintenance of tuning, and the like.

Surprisingly it has been found that single axial-mode operation ispossible in a high power OPS-laser in accordance with the presentinvention apparently independent of the physical length of theresonator, at least up the 25 cm length of the above exemplifiedinventive laser. It is believed that this length could be significantlyfurther extended, for example, up to 100 cm or greater, while stillmaintaining single-axial-mode operation without any special provisionfor providing such operation.

It is believed, without being limited to a particular theory, that thissurprising discovery may be explained as follows. In all above-describedembodiments of the present invention, gain-structure 34 of OPS-structure32 is located precisely at the end of the “fundamental-resonator”, aposition in which all possible axial-modes of operation (standing waves)have a node, i.e., are exactly in phase.

An OPS gain-structure of the present invention, is only about sevenwavelengths “long”. The resonator itself may be tens of thousands oreven hundreds of thousands of wavelengths long. Accordingly, even widelynumerically separated modes (i.e., N and N±m where m is an integer) donot become appreciably out of phase within the gain-medium.

Now, the OPS gain-structure, with half-wavelength-spaced QW layers onlyprovides gain in narrow region one-half-wavelength apart. Accordingly,as the actual wavelengths of adjacent possible modes differfractionally, at most, by only a few millionths of a wavelength, or afew hundredths of the width of the QW layers, all possible modes haveabout an equal chance to extract the available gain. That mode whichbegins to oscillate first wins the “mode-competition” and deprives theothers of the gain needed to oscillate, thereby forcing singleaxial-mode operation.

To ensure true single-mode (single-frequency or TEM₀₀) operation in anOPS-laser in accordance with the present invention, it is preferable todesign resonators, wherein, as exemplified above, the pumped-area(pump-spot) and fundamental spot-size at OPS-structure 32 are about thesame, i.e., the ratio of pump spot-size to mode-size is about unity(1.0) or less. This provides that there is insufficient gain availableoutside the TEM₀₀ spot-size to support other transverse-modes ofoscillation of different frequencies while still deriving maximum gainfrom the pumped area.

Above-described OPS-lasers in accordance with the present invention havebeen described in terms of having physically long resonators, it beingunderstood here, that what is referred to as resonator-length is theaxial-length or distance between resonator end-mirrors. One skilled inthe art, however, will recognize that increasing the physical resonatorlength is not the only way to provide for a large mode-size in thegain-structure 34 of OPS-structure 32 for increasing power.Incorporating a suitable optical system within a resonator can produce alarge “equivalent length” for the resonator, even when the actual(physical) length of the resonator is relatively short. This may be usedto provide a large spot-size at an OPS-structure in accordance with thepresent invention with a resonator having a relatively short physicallength. An example of such a resonator arrangement used in a prior-art,flashlamp-pumped Nd:YAG laser, employing a telescopic arrangement oflenses in the resonator, is described in a paper by Hanna et al., Opt.Quantum Electron. 13, 493 (1981).

Ray optics considerations indicate, however, that in any such complexresonator, wherein a mirror-structure 30 of an OPS-structure 32 forms atone extremity of the resonator, the complex resonator is equivalent (asseen from mirror-structure 30) to a simple resonator consisting of amirror (generally curved) placed at a specific distance frommirror-structure 30 of the OPS-structure 32. In other words, anyarbitrarily complex resonator can always be reduced, for the purpose ofdetermining mode-size at gain-structure 34 of OPS-structure 32, to anequivalent simple resonator having mirror-structure 30 of OPS-structure32 at one end thereof, and a curved mirror at the other end thereof.Accordingly, in long-resonator-type OPS-lasers in accordance with thepresent invention there can be defined an equivalent or opticalresonator-length necessary to provide a desired mode-size atOPS-structure 32. This optical length definition encompasses therelatively simple resonator structures described above, wherein theoptical length is close to the physical length as well as any morecomplex arrangements for shortening physical length while maintaining asufficiently large optical length. A preferred optical length for theresonator of an OPS-laser in accordance with the present invention is atleast about 5 cm and preferably greater than about 10 cm.

The most straightforward way to increase the mode or spot-size at anOPS-structure 32 in an OPS-laser in accordance with the presentinvention is to increase the optical length of the resonator by simplyincreasing the physical length of the resonator, with an appropriatechoice of the radius of curvature of resonator mirrors. The beamspot-size required for the generation of multi-watt radiation in theabove-exemplified laser 20 of FIG. 1 was obtained by employing aresonator length of 25 cm, which, even though much larger than resonatorlengths of prior art OPS-lasers, is still small enough to allow forconvenient packaging of the laser while still allowing operation in asingle axial-mode.

Turning next to thermal management issues, as discussed briefly above,thermal-management of OPS-structure 32 in an OPS-laser in accordancewith the present invention is particularly important. Accordingly, adiscussion of some important aspects of the thermal management is setforth below with reference to FIG. 8. FIG. 8 depicts, in detail,OPS-structure 32 and heat-sink 36 on which it is bonded.

The thermal-management problem may be summarily defined as follows.Pump-light 42 is absorbed in gain-structure of OPS-structure 32. Thatportion of the absorbed pump-light which does not generateelectrical-carriers, i.e., does not provide laser-radiation, generatesheat in gain-structure 34. This heat must be conducted away by heat-sink36 at a rate sufficient to keep the temperature of gain-structurepreferably below a temperature of about 90° C. It should be noted herethat this temperature is merely provided for guidance and should not beconsidered critical or limiting.

Mirror-structure 30 impedes the conduction of heat from gain-structure32 to heat-sink 36. As discussed above, in a preferred, inventiveheat-sink arrangement, OPS-structure 32 is bonded, by a solder layer110, to a layer 112 of synthetic diamond, preferably having a thicknessof about 0.3 mm. Diamond-layer 112, in turn is bonded, by another solderlayer 114, to a copper-bodied microchannel-cooler 116.

In developing a heat-sink configuration for OPS-structure in accordancewith the present invention, consideration was first given to theheat-sink properties of massive substrates of copper and diamond. Fromcalculations considering the pump-power density provided by the aboveexemplified 34 W of pump-power delivered directly on the surface of amassive (essentially infinite in extent) copper heat-sink it wasestimated that surface temperature on such a heat-sink would be 110° C.In practice, the 34 W of pump power would not be delivered directly tothe surface but would be impeded by the materials of mirror-structure30. These materials have a relatively low thermal-conductivity.Accordingly, the temperature of gain-structure 34 could be expected tobe about 15° C. higher than the highest temperature at the surface ofthe heat-sink.

From calculations considering the pump-power density provided by theabove exemplified 34 W of pumping delivered directly on the surface of amassive CVD-diamond (having a thermal-conductivity 2.5 times higher thancopper) heat-sink, it was estimated that the surface temperature on sucha heat-sink would be 45° C. This would be a tolerable temperature forgain-structure 34 of OPS-structure 32. However, a diamond (evensynthetic) sufficiently large to be considered of infinite extent wouldbe prohibitively expensive, if at all obtainable.

Giving consideration to a microchannel-cooler, it was determined thatthe surface temperature on such a cooler provided by the aboveexemplified 34 W of pumping delivered directly thereto would be about110° C. It was then determined that if a relatively thin layer ofsynthetic diamond were bonded to the same microchannel-cooler, thesurface temperature (on the diamond layer) would be only 55° C. degrees,i.e., comparable with the calculated temperature for the above-discussedhypothetical massive diamond heat-sink. This determination wassignificant in enabling the pump-power densities required for OPS-lasersin accordance with the present invention to be safely delivered toOPS-structure 32.

On a prima facie consideration, it may appear that at a constantpump-power density, the above-exemplified temperatures in OPS-structure32 may be expected to be remain the same as the pumped-area thereon isincreased, making power-scaling beyond the above identified levelsmerely a matter of increasing resonator length. On closer investigation,however, this proves not to be the case.

By way of illustration of the pumped-area scaling problem, FIGS. 9 and10 depict computed isothermal contours (assuming radial symmetry) inrespectively a relatively-massive (1.0 mm-thick) copper heat-sink and ina 0.3 mm-thick CVD-diamond layer bonded on a 0.7 mm-thickcopper-heat-sink. The isothermal contours represent response to auniform surface heating at a rate Q of 180 Watts per square mm (W/mm²)within a circle H of 0.25 mm radius (corresponding to theabove-exemplified pump-power density on OPS-structure 32). Isothermalcontours are at 5° C. intervals. The terminology “relatively massive”,as used above, means in relationship to the dimensions of OPS-structure32.

It can be seen from both FIGS. 9 and 10 that the temperature rises fromedge to center of the heated (pumped) area. Increasing the pumped-areaat the same pump-power density, accordingly, would increase the peaktemperature of the OPS-structure. Fortunately, however, this increase isproportional to the square-root of the increase in area. This allowssome trade-off between pump-power density and increasing pumped-area.

The effectiveness of the CVD-diamond layer, of course is also evidencedby the lower peak temperature of between 45° C. and 50° C. achieved withthe diamond layer (see FIG. 10) compared with a peak temperature ofabout 100° C. without the CVD-diamond layer (see FIG. 9).

Synthetic diamond in single crystal form has twice thethermal-conductivity of the CVD form. Accordingly, by substituting asingle crystal diamond layer for the CVD-diamond layer a further surfacetemperature reduction of about 20° C. could be expected. Syntheticdiamond in CVD and single crystal form is available from SumitomoElectric Industries (USA) Ltd, of New York, N.Y.

The high thermal-conductivity of the above discussed inventivearrangement of heat-sink 36 ensures not only low temperature ingain-structure 34 of OPS structure 32, but also low transversetemperature gradients in the plane of the OPS-structure. Transversetemperature gradients cause, through temperature dependence of the indexof refraction of the OPS material, transverse variation of the opticalpath seen by the fundamental-radiation on propagation in theOPS-structure. Such gradients manifest themselves in a solid-state gainmedium as “thermal-lensing” which means that the gain-medium behaves, ina first order approximation, as a positive spherical lens. Thisphenomenon is well known in prior-art DPSS lasers, and complicates thedesign of resonators for such lasers, as these resonator designs musttake into account both the thermal-lensing and its variation as thepump-power level is changed. By way of example, a typicalneodymium-doped yttrium vanadate (Nd:YVO₄) gain-medium crystal canexhibit thermal-lensing as high as seven diopters under typical pumpconditions.

In considering the design of heat-sink 36 it was recognized that carefuldesign could lead to exceptionally low values for the thermal-lensing inan OPS-laser in accordance with the present invention compared withvalues encountered in prior-art DPSS lasers. By way of example, in theabove-described example of laser 20 (FIG. 1), employing a CVD diamondlayer on a microchannel cooler as a heatsink, a thermal lensing ofsignificantly less than one diopter was measured. The use of synthetic,single-crystal diamond instead of CVD diamond would reduce this alreadysmall thermal lensing by a factor of two. Such low thermal-lensingsimplifies considerably the design of the laser resonator. Generally, indesigning a resonator for an OPS-laser in accordance with the presentinvention, it may be assumed that there is will be no thermal lensing.

Turning now to the heat-transfer-impeding effect of mirror-structure 30of OPS-structure 32, in the example of the inventive OPS-laser discussedabove, the total thickness of mirror-structure 30 of OPS-structure 32 isabout 4.1 μm. Computations, based on this total thickness; bulkthermal-conductivity values of GaAs and AlAsP; and the structure ofcooled-substrate 36 as depicted in FIG. 6, indicated that because of theheat-transfer-impeding effect of mirror-structure 30, the temperature ofgain-structure 34 would be between about 18° C. and 20° C. higher thanwould be the case, were the gain-structure directly-bonded to diamondlayer 112 of cooled-substrate 36. Accordingly, a substantial portion ofthe potential reduction in surface temperature offered by the bondeddiamond layer is sacrificed to the mirror-structure. From experimentalobservations of the shift of the fluorescence spectrum of gain-structure34 under the operating conditions of the above described practicalexample of inventive OPS-laser, it has been determined thatgain-structure 34 is at a temperature of about 90° C.

It is clear, that an improvement in the thermal-conductivity ofmirror-structure 30 of OPS-structure 32 could allow a higher pump-powerto be delivered to gain-structure 32. This would allow further increasesin output-power. Some approaches to improving this thermal-conductivityare set forth below.

An advantage of growing OPS-structure 32 in the manner described aboveis that it is neither necessary that mirror-structure 32 beepitaxially-grown, nor necessary that it be formed entirely, if at all,from semiconductor materials. Given this flexibility, it is possible todesign mirror-structures 30 of OPS-structures 32 in accordance with thepresent invention using a wide selection of materials for forming highand low refractive index layers. Accordingly, values of absoluterefractive index of the materials, and thermal-conductivity of thematerials, can be selected such that such that the thermal-conductivityin the axial direction of the mirror-structure (as a whole) ismaximized. Clearly, of course, the materials selected must have minimalabsorption (be highly transparent) for the fundamental-wavelength suchthat mirror-structure is capable of providing maximum reflectivity forthe fundamental-wavelength. Maximum reflectivity in this regard may bedefined as a value preferably about 99.9 percent or greater.

One option for increasing thermal-conductivity of mirror-structure 30 isto substitute a layer of a highly reflective metal such as aluminum,silver, gold, copper or magnesium, for several high and low refractiveindex layer-pairs of transparent materials, and to allow an absorptionin the metal layer of about 0.1 percent. Dielectric multilayerovercoated aluminum mirrors of similar structure, for providinginexpensive high-efficiency reflectors for optical scanners, copiers andthe like, are often referred to in the optical-coating art as “enhancedmetal-mirrors”.

A metal layer in a mirror-structure 30 in accordance with the presentinvention would be the last-deposited (mirror) layer and would befurthest from the gain-structure. As this metal layer, in a completedOPS-structure 32, would be in direct contact with the heat-sink, it isbelieved that at least 0.1 percent absorption would be tolerable.

By way of example, using the GaAs and AlAsP materials exemplified above,with a 1000 Å layer of gold as the final layer, 99.9% reflectivity couldbe provided by a structure having only nine pairs of GaAs and AlAsPlayers. Such a structure would have a total thickness of only about 1.47μm, i.e., about one-third the thickness of the mirror-structure of theabove-described example. Accordingly up to a three-fold increase inthermal-conductivity of the mirror-structure as a whole can beanticipated.

It is believed that further reductions in thickness could be possibly beobtained by using a material having a much lower refractive index thanAlAsP as a low refractive index material, for example, barium fluoride(BaF₂) having a refractive index of about 1.48. Using a 1000 Å-thicklayer of gold as a final layer, 99.9% reflectivity could be provided bya mirror-structure 30 having as few as two pairs of GaAs and BaF₂layers. Such a mirror-structure would have a total thickness of onlyabout 0.54 μm, i.e., about one-eighth the thickness of the OPS-structureof the above described example. What is gained in thermal-conductivityby the total thickness reduction, however, will be at least partiallyoffset by the lower conductivity of BaF₂ compared with AlAsP and thegreater physical thickness of BaF₂ required to provide one-quarterwavelength optical thickness at the fundamental-wavelength.

Such “enhanced metal-mirrors” may be particularly useful inOPS-structures which have fundamental emission at wavelengths shorterthan about 0.7 μm where semiconductor materials have too great anabsorption to be used effectively in multilayer reflectors. For thesewavelengths, use must be made of dielectric materials such as titaniumoxide (TiO₂) and niobium oxide (Nb₂O₅) to provide high index layers, incombination with materials such as silicon dioxide (SiO₂) as a low indexlayer. It is useful to note that a mirror using only a combination ofTiO₂ and SiO₂ layers, having the same reflectivity as a mirror made fromonly GaAs and AlAsP layers, would have a total thickness about fiftypercent greater.

It should be noted that, depending on the selection of a highlyreflecting metal for an “enhanced-metal” type mirror-structure, and thetype of bonding material selected for bonding the OPS-structure toheat-sink 36, it may be necessary to add a layer, preferably of asuitable metal, on the reflective metal layer to prevent that layer fromreacting, alloying or dissolving in the bonding material. As thisadditional layer, would be (as far as incident laser radiation isconcerned) “behind” the reflective metal layer it is not important thatit be highly reflecting.

As discussed above, OPS-structures such as OPS-structure 32 are grownover essentially the entire area of a semiconductor wafer. After theOPS-structure is grown, the wafer is diced into chips of OPS-structure.A prior-art-preferred dicing method involves scribing and breaking thewafer to form the chips. Breakage takes place, cleanly and smoothly,along a crystalline plane of the wafer and the epitaxially grownOPS-structure thereon. Referring now to FIG. 11, an OPS-structure chip(designated here, for consistency by reference numeral 32) formed asdescribed above, has what may be termed as an emission face 119 ongain-structure 34 and has smooth end-faces 120 in resulting from thescribe-and-break dicing method. End faces 120 are at right angle toemission-face 119 and have a relatively-high Fresnel reflectivity (about30%) due to the relatively-high refractive index of semiconductormaterials of the gain-structure. At operating powers contemplated forOPS-lasers in accordance with the present invention, absent anypreventive arrangements, laser action can take place laterally(indicated in FIG. 11 by arrows B) in resonators formed by parallelpairs of end-faces 120.

During operation, such parasitic lateral oscillation would robgain-structure 34 of OPS-structure 32 of gain which would otherwise beavailable for providing laser-action in the intended, surface-emittingdirection (indicated in FIG. 11 by arrows C). Certain measures may betaken to suppress such parasitic lateral oscillation. One such measurecomprises employing a dicing method wherein the wafer is cut or scribedcompletely from one side to the other with a diamond-scribe,diamond-saw, or like abrasive-cutting device. Following such scribing,end-faces 120 will be sufficiently rough or irregular that they areineffective as mirrors. Accordingly, parasitic lateral oscillation willeffectively suppressed. Other methods of roughening the end faces arenot precluded.

If end-faces 120 are left smooth as a result of scribe-and-break dicing,coating the end-faces with a simple antireflection coating of materialhaving a lower refractive index than the end-faces, such as aluminumoxide (Al₂O₃) or yttrium oxide (Y₂O₃), can reduce the reflectivity ofend-faces 120 to a level where they no longer are effective as mirrors,thereby effectively suppressing parasitic lateral oscillation.

Yet another means of suppressing parasitic lateral oscillation is toensure that the longest linear dimension (end-face length) of chip 32(OPS-structure 32) is significantly larger than the pumped-area 122. Atpower levels contemplated for OPS-lasers in accordance with the presentinvention, the length of an end-face 120 is preferably greater thanabout three-times the diameter of the pumped area. Intrinsic absorptionof the unpumped OPS-structure materials and the increased distancerequired for radiation to travel between end-faces 120 can providesufficient loss to effectively suppress parasitic lateral oscillationeven if end-face 120 are smooth.

Deliberately increasing absorption of the materials of gain-structure 34outside the pumped area for suppressing parasitic lateral oscillation isnot without the bounds of possibility. This may be accomplished, forexample, after growth of OPS-structure 32, by selective doping or ionimplantation. Clearly, any of the above-described measures for reducingparasitic lateral oscillation may be use alone or, where appropriate, incombination.

Turning now to selection of frequency-conversion materials forOPS-lasers in accordance with the present invention, as noted above, therange of wavelengths available from external cavity OPS-lasers may beextended by frequency multiplication, doubling or mixing. This ispreferably accomplished by intracavity (IC) frequency-conversion(harmonic generation), in which an optically-nonlinear crystal isincluded in a resonator formed between the mirror-structure of anOPS-structure and a (external to the structure) conventional mirror. Aparticularly preferred optically-nonlinear crystal material for ICfrequency converted OPS-lasers in accordance with the present inventionis LBO.

Because of the limited power available in prior-art OPS-lasers, it hasbeen the practice in such lasers to use an optically-nonlinear materialwith as high a frequency conversion-efficiency as possible, typically,KNbO₃. KNbO₃, however, has certain disadvantageous properties that makeit a less-than-ideal choice for use in a high-power frequency-doubledlaser in accordance with the present invention. These disadvantageousfeatures are a certain fragility, and a limited spectral-acceptancerange. The spectral-acceptance range of an optically-nonlinear materialis that range of wavelengths which would be frequency-converted for acrystal of the material.

FIG. 12 depicts the computed, normalized spectral-acceptance of KNbO₃,and of the significantly-more-durable, but less efficient, LBO. Each isassumed to be in the form of a 5 mm-long crystal. Also depicted is thegain-bandwidth of active layers of InGaAs having a composition selectedto provide laser action around 976 nm. It can be seen that thespectral-acceptance of the KNbO₃ is about one-tenth that of the LBO, andis very narrow by comparison with the gain-bandwidth of the activelayers.

Now, as the frequency-doubling mechanism of an intracavityoptically-nonlinear crystal represents a loss for thefundamental-wavelength, absent any other constraint, the resonator willtend to shift (hop) its oscillation wavelength to a wavelength wherethere is no loss. When this happens, frequency-doubled output will dropcatastrophically, if not cease altogether, and fundamental power willsurge correspondingly.

Wavelength-hopping will result in a very noisy frequency-convertedoutput of lower average power than would be the case in the absence ofwavelength-hopping. This wavelength-hopping can be limited by includingin the resonator a highly wavelength-selective element such as abirefringent filter, an etalon, or the like. Any such device having thenecessary selectivity, however, would introduce significant resonatorlosses, thereby limiting output-power. Either way, this would mean thatthe nominally high conversion-efficiency of the KNbO₃ may not berealized, in practice, in its full measure. Further, thewavelength-selective device would probably require an active measuresuch as temperature control to satisfactorily stabilize itspeak-transmission wavelength.

The LBO material, having the much wider acceptance would still not becompletely immune from wavelength-hopping, without thewavelength-restraining benefit of an intracavity wavelength-selectivedevice. Such a wavelength-selective device in the case of LBO, however,can be ten-times less selective than would be required for the KNbO₃;would not require active stabilization; and would not introducesignificant cavity losses.

For completeness, it is pointed out here that the spectral-acceptance ofan optically-nonlinear crystal is inversely proportional to the lengthof the crystal. The second-harmonic-generating (SHG) efficiency of sucha crystal is roughly proportional to the square of a material constantd_(eff), multiplied by the square of the length of the crystal. As thevalue of d_(eff) for KNbO₃ is about ten-times greater than the value ofd_(eff) for LBO, then, in theory at least, a 0.5 mm-long KNbO₃ crystalwould have the same peak SHG-efficiency and spectral-acceptance as a 5.0mm-long LBO crystal. It is believed, however, that because of thefragility of the KNbO3, a 0.5 mm long KNbO3 crystal may be somewhatimpractical to fabricate and handle. The use of KNbO3 crystals inresonators in OPS-laser resonators in accordance with the presentinvention, however, is not precluded.

Generally, for reasons discussed, above LBO and chromium lithiumtriborate (CLBO) are preferred for frequency-doubling and,alternatively, for frequency tripling. BBO is preferred for frequencyquadrupling and, alternatively, for frequency tripling. For doubling ortripling wavelengths shorter than about 670 nm, optically nonlinearmaterials including strontium barium borate (SBB0), strontium borate(SBO), and barium zinc borate (BZBO) are believed to be effective. Theseoptically-nonlinear material preferences should not, however, beconsidered as limiting IC frequency-converted lasers in accordance withthe present invention.

TABLE 1 Fundamental Bragg Wavelength Mirror QW/Separator Substrate (nm)1 In_(x)Ga_(1−x)As/GaAs_(y)P_(1−y) GaAs  900-1050 2In_(x)Ga_(1−x)P/In_(y)Ga_(1−y)As_(z)P_(1−z) GaAs 700-900 3InAs_(x)P_(1−x)/InGa_(y)P_(1−y) InP  930-1800

TABLE 2 Bragg Mirror Materials Structure High Refractive-Index LowRefractive-Index Number Layers Layers 1 GaAs AlAs_(x)P_(1−x) 2In_(x)Al_(1−x)P Al_(y)Ga_(1−y)As 3 Dielectric Dielectric

Continuing now with a description of preferred materials for high-powerOPS-structures in accordance with the present invention, TABLE 1 listspreferred QW layer and separator-layer materials for gain-structure 30.Each set of materials is associated with a particular mirror-structure30 (Bragg mirror) number, substrate and fundamental-wavelength range.Details of Bragg mirror, composition are given in TABLE 2.

The structures represented in TABLES 1 and 2 represent a departure fromprior-art-preferred structures for the same wavelength ranges, in regardto both mirror-structures and gain-structures. In TABLES 1 and 2compound composition proportions (subscripts) x, y, and z all have avalue between 0.0 and 1.0. In each case, materials (compounds) areselected such that the stress of the OPS-structure as a whole is as lowas practically possible, by ensuring that the stress-thickness productof all tensile strained layers and all compressive strained layers isabout of equal magnitude. This is of particular importance inOPS-structures in accordance with the present invention which aresubject to the stress of high operating temperatures, andcorrespondingly large temperature-cycles between periods of operationand inactivity.

The OPS-structure (see FIG. 2) of the above-exemplified ICfrequency-doubled OPS-laser in accordance with the present invention isan example of the first-listed family of structures wherein GaAs“transition” layers between the QW and separator layers, alleviate thestress-difference which would otherwise exist between these layers. Thegain-structure of the OPS-structure of FIG. 2 can be generallycategorized as being formed from components of the III-V quaternarysystem InGaAsP wherein the substrate material is a binary III-V compoundof the components (here GaAs), and wherein the QW layers are formed froma one possible ternary III-V compound including both components of thesubstrate material, and the separator layers are formed from the otherpossible ternary III-V compound including both components of thesubstrate material. Type 3 structures of TABLE 1 (on InP substratematerial) also fall into this category. This is believed to provide theoptimum stress compensation for OPS structures formed from thisquaternary system. For type 3 structures of Table 1 a mirror-structureincluding dielectric materials is preferable. One preferred structureincludes high refractive index layers of zinc selenide (ZnSe) and lowrefractive index layers of aluminum oxide. This combination providesabout the same reflectivity per number of layer-pairs as the combinationof TiO₂ and SiO₂ used in general coating applications in the prior-art,but with a lower total physical thickness and higher thermalconductivity. It is emphasized here that the mirror structures of TABLE2 are preferred structures for use with QW/separator structures of TABLE1, the use of other mirror structures with the QW/separator structuresof TABLE 1 in OPS-lasers in accordance with the present invention is notprecluded.

Further, from the description of inventive OPS-lasers provided above,those skilled in the art may select other OPS-structures for providingfundamental-radiation at wavelengths between about 425 nm and 1800 nmwithout departing from the spirit and scope of the present invention.Such structures may include, by way of example, QW or active layersselected from a group of semiconductor compounds consisting of,In_(x)Ga_(1−x)As_(y)P_(1−y), Al_(x)Ga_(1−x)As_(y)P_(1−y), andIn_(x)Ga_(1−x)N where 0.0≦x≦1.0 and 0≦y≦1. This list, however, shouldnot be considered as limiting the present invention. Second, third andfourth harmonic wavelengths, of course, would be between about 212 and900 nm; 142 and 600 nm; and 106 and 225 nm. Performance in the shorterwavelength half of the latter range may be somewhat limited by theavailability of suitable optical materials.

Turning next to a discussion of further aspects of increasingoutput-power in OPS-lasers in the present invention, in embodiments ofOPS-lasers described above, only one OPS-structure 32 is included.Several approaches to increasing output-power to beyond the levelsachieved or confidently predicted have been discussed without regard tothis. It will be evident to one skilled in the art from thesediscussions, however, that exploiting the above-discussed approachesalone may lead to a point of diminishing returns. Should this prove tobe the case, or simply as an alternative, output-power (fundamental orharmonic) may be increased by incorporating at least a secondOPS-structure in a suitably-configured resonator. A description of onesuch resonator is set forth below.

Referring to FIG. 13, a further embodiment 128 of an ICfrequency-doubled OPS-laser in accordance with the present invention isdepicted. Laser 128 comprises a tightly-folded resonator 130 includingtwo OPS-structures, 32 and 32X. Resonator 130 is terminated at one endthereof by mirror-structure 30 of OPS-structure 32 and at the other endthereof by a mirror 28. Resonator 130 includes a birefringent filter 52for reasons discussed above. The OPS-structures are mounted onabove-described heat-sink assemblies 36. Resonator axis 132 is foldedonce by mirror-structure 30X of OPS-structure 32X, and again by afold-mirror 26. An optically-nonlinear crystal 50, arranged forfrequency-doubling, is located in arm 130A of resonator 130, i.e.,between fold-mirror 26 and end mirror 28. Mirror 28 is coated formaximum reflectivity at the fundamental-wavelength. Mirror 26 is coatedfor maximum reflectivity at the fundamental-wavelength and maximumtransmission at the 2H-wavelength.

It should be noted here that the tight folding of the resonator isselected to maintain as close to normal incidence as possible onOPS-structure 32X. This is to minimize interference in gain structure34X between counterpropagating fundamental beams. Such interference canlead to the formation of interference-fringes (a sort of “lateralspatial hole-burning”) in the plane of the QW layers, thereby limitinggain-extraction from these layers. In this regard, it is also preferableto adjust spacing of the QW layers in gain-structure 34X and thethickness of layers of mirror-structure 30X to compensate for theeffective refractive index change due to non-normal incidence, therebymaintaining effective half-wavelength spacing of the QW layers.

Continuing with reference to FIG. 13, a further difference between laser128 and other above described OPS-lasers in accordance with the presentinvention is the manner in which pump-light is delivered to theOPS-structures. In laser 128, pump-light is transported along fibers 40as in other above-described embodiments. Fold-mirror 44 andfocusing-lenses 46 and 48 of these other embodiments, however, arereplaced with an assembly of two radial-gradient-index lenses 146 and148. Such lenses are often referred to generically as “selfoc”. It hasbeen found such lenses can provide the same optical performance aslenses 46 and 48 in the above-described example of laser 20. In oneexample lens 146 is a Part No. 06LGS216 available from Melles Griot Incof Irvine Calif. Lens 148 is a Part No. SLW180-020-083-A2. This selfoclens arrangement provides convenient access for multi-fiber(fiber-bundle) pumping in the case of tightly-folded resonators such asresonator 28. The arrangement may be used in any other above-describedembodiment simply to reduce the volume of pump-light delivery optics orto permit power to be delivered by more fibers than would be possiblebecause of the bulk of the conventional focusing-optics.

From the foregoing description, those skilled in the art to which thepresent invention pertains may devise OPS-lasers including more than twoOPS-structures without departing from the spirit and scope of thepresent invention. Such OPS-lasers may be configured for deliveringoutput-radiation at the fundamental wavelength of the OPS-structures orat harmonic wavelengths thereof.

From the description of laser 128 it will also be evident to thoseskilled in the art that a folded laser-resonator may be formed betweentwo external mirrors, and include one or more OPS-structures 32 only asfold mirrors. Such a folded laser-resonator is depicted in FIG. 14.Here, an OPS-laser 140 in accordance with the present invention includesa laser-resonator 142 terminated by mirrors 28F and 144 andtightly-folded (for reasons discussed above) by mirror structure 30 ofOPS-structure 32. Heat-sink and optical-pumping arrangements are thesame as described above for laser 128. Laser resonator optionallyincludes a birefringent filter 52.

The laser-resonator arrangement of laser 140 may be useful if it desiredto configure a laser-resonator which is terminated by two non-flatmirrors. It will be evident from the arrangement of laser 140 thatsimilar arrangements wherein a resonator is not terminated at one endthereof by a mirror-structure of an OPS-structure may be devised for anyother above described embodiments of OPS-lasers in accordance with thepresent invention.

In summary, several embodiments of high-power, IC frequency-converted,OPS-lasers in the accordance with the present invention have beendescribed above. Generally, embodiments of the inventive laser cangenerate second, third or fourth harmonic output-power from fundamentalradiation at a power of 100 mW or greater, and even at 1 W or greater.These high-harmonic output powers can be achieved in single axial modeor TEM₀₀ operation with a beam-quality of less than about twice thediffraction-limit. Particularly notable is the ability of the inventivelasers to generate UV output-radiation at these high output-powers, moreparticularly, at better than one percent pump-to-harmonic efficiency bysecond-harmonic generation from fundamental radiation in a 700 nm to 800nm wavelength ranges.

The inventive OPS-lasers, can provide a means of generating wavelengths,in a true CW mode of operation, which can closely match the optimumwavelength for many laser applications, in fields such as medicine,optical metrology, spectroscopy, optical lithography, and precisionlaser machining. By way of example, an inventive OPS laser canefficiently produce CW output radiation in the 590 to 600 nm wavelengthrange by second-harmonic generation from an OPS-structure emitting inthe 1180 to 1200 nm range. 590 to 600 nm is advantageous for ophthalmicsurgical application. This wavelength range can not be efficientlygenerated by prior-art lasers. The ability to generate high-power UVradiation is applicable in particular in areas such as direct writing orpatterning of printed circuit boards or conventional. UV plates. In suchapplications the inventive OPS-lasers offer not only power, efficiencyand beam quality, but also the ability for many applications to “tailor”an output wavelength to an absorption-peak of material to be exposed,cut, ablated, heated, photochemically altered or otherwise treated,thereby reducing the absolute power required for the applications. Fromthe above presented description of the OPS lasers those skilled in theappropriate arts may conceive many other uses for the inventive laserswithout departing from the spirit and scope of the present invention.

The present invention is described and depicted herein with reference toa preferred and other embodiments. The present invention is notrestricted, however, to those embodiments described and depicted.Rather, the invention is limited only by the claims appended hereto.

1. A laser, comprising: a resonant cavity formed by two end mirrors,wherein the optical length of the resonant cavity is at least 25 cm; atleast two separate and independent OPS-structures each OPS-structurehaving a gain structure with an emitting surface, each of said gainstructures including a plurality of active layers having a compositionselected to provide emission of a laser radiation beam at apredetermined fundamental wavelength when optical pump light is incidenton said emitting surface of said gain structure, each of said gainstructures surmounting a mirror structure and with said mirror structuresurmounting a heat sink; and an optical arrangement for delivering pumplight to an area on said emitting surface of each of said gainstructures, thereby causing said cavity to deliver fundamental outputlaser radiation at a power greater than 5 Watts in a substantiallysingle longitudinal mode and wherein the pump light is spatiallycontinuous over the area on said emitting surface where the pump lightis delivered.
 2. A laser as recited in claim 1, wherein one of said endmirrors is a curved output coupler.
 3. A laser as recited in claim 1,wherein one of said end mirrors is flat and highly reflective at thefundamental wavelength.
 4. A laser as recited in claim 1, wherein one ofsaid end mirrors is defined as a mirror of one of the OPS structures. 5.A laser as recited in claim 1, wherein said cavity further includes awavelength-selective element.
 6. A laser as recited in claim 1, whereinsaid cavity further includes a birefringent filter.
 7. A laser asrecited in claim 1, wherein said cavity further includes an etalon.
 8. Alaser as recited in claim 1, wherein at least one of the mirrorstructures on the OPS structures functions to fold the cavity.
 9. Alaser as recited in claim 1, wherein the radiation exiting the cavityhas a mode quality (M²) about 2.0 or less.
 10. A laser as recited inclaim 1, size of the area on the gain structure which is pumped by thepump light is selected to substantially match the size of thecirculating radiation beam at that location.
 11. A laser as recited inclaim 1, wherein optical arrangement for delivering the pump light toeach OPS gain structure includes two laser diode arrays.
 12. A laser asrecited in claim 11, wherein the light from the two laser diodes arraysare directed to the gain structure at a non-normal angle of incidence intwo opposing directions so as not to interfere with the fundamentalradiation circulating in the cavity.
 13. A laser as recited in claim 1,wherein said heat sink includes a metal body and further comprises adiamond layer between the mirror and the metal body.
 14. A laser asrecited in claim 13, where said diamond layer is a CVD diamond layer.15. A laser as recited in claim 1, wherein said heat sink includes ametal body and further comprises a layer formed from a diamond compositeand located between the mirror and the metal body.
 16. A laser asrecited in claim 1, wherein the surface of the gain structures of theOPS-structures that receives the pump light is provided with ananti-reflection coating.
 17. A laser as recited in claim 1, wherein saidcavity includes a means for tuning the fundamental output wavelength.18. A laser as recited in claim 17, wherein said means for tuning theoutput wavelength is a rotatable birefringent filter.